Fatigue Crack Growth Process Research Articles

Failure behavior of aerial bomb lifting lug under variable amplitude loading: Failure analysis and life prediction

The lifting lug is one of the most important structures of aerial bomb, which mainly bears the variable amplitude cyclic loading during the working process. In this paper, the failure analysis of several lifting lugs which failed in fatigue test was carried out. Firstly, through the mechanical analysis, fatigue cumulative damage analysis and fracture surface analysis of the lifting lug structure, it was determined that the fracture failure was not caused by the issue of structural design, but by the Al2O3 inclusion defect in the casting process. Next, the crack source defect was equivalent to the initial crack, and the fatigue full-life was predicted by the method of fatigue crack growth (FCG). The FCG process was divided into two stages: small crack growth stage and long crack growth stage for life prediction respectively. Finally, the validity of this method was verified by comparing with the experimental results, and it was suggested to adopt advanced lifting lug casting and detection technology to prevent similar faults.

Probabilistic modeling of fatigue crack growth and experimental verification

Stochastic fatigue crack growth (FCG) modeling is vital for fatigue reliability and durability analyses of engineering components. To characterize the statistical properties of FCG process, the Yang–Manning model was introduced to calculate the crack exceedance probability and fatigue life distribution to reach any crack size; however, it is only applicable for failure probability analyses of pre-cracking specimens. According to this, a new probabilistic fatigue life model considering the distribution of initial crack sizes was proposed. Experimental data of Al 2024-T3 alloy and 30NiCrMoV12 steel were utilized for model validation and comparison. Results show that the proposed probabilistic model works well for FCG analysis with independent cracks. In addition, the Yang–Manning model was extended to consider the crack coalescence issue under multiple cracks condition, which shows a sound conformity to experimental data.

Fatigue crack growth mechanisms in powder metallurgy Ni-based superalloys—A review

Powder metallurgy (PM) Ni-based superalloys are widely used for aeroengine turbine disc applications due to their excellent mechanical properties and good corrosion resistance at elevated temperatures. Understanding the fatigue crack growth (FCG) mechanisms of PM Ni-based superalloys is important for both disc alloy development and life prediction of disc components in these advanced aeroengines where damage tolerance design prevails. FCG in PM Ni-based superalloys is a complicated function of microstructure, temperature, loading conditions and environment and is usually a consequence of the synergistic effects of fatigue, creep and environmental damage. In this review, the mechanisms controlled by microstructural features including grain size, grain misorientation, γ′ size and distribution on short and long FCG behaviour in PM Ni-based superalloys are discussed. The contribution of creep and environmental damage to FCG has been critically assessed. The competing effects of mechanical damage (i.e. fatigue and creep) and environmental damage at the crack tip are microstructure-sensitive, and usually results in transition between transgranular, mixed-trans-intergranular and intergranular FCG depending on the contribution of environmental damage to FCG processes.

Fatigue crack growth prediction for small-scale yielding (SSY) and non-SSY conditions

A model to predict fatigue crack growth (FCG) for small-scale yielding (SSY) and non-SSY conditions where J-integral applies is established. The ΔJ-characterized elastic-plastic fracture mechanics (EPFM) analysis is performed based on a rearranged FCG process to achieve the crack-tip geometry and stress state. The root stress of blunted crack and the FCG rate are calculated from the cycle-wise local and global energy balance. To consider effects of geometry, load level and strain hardening, a correction factor is further developed in the governing equation of global energy balance. The model is validated using the test data of several types of metals.

Experimental and numerical investigation on fatigue crack growth behavior of commercial pure titanium under I-II mixed mode loading at negative load ratios

With the increase of design and application parameters for pressurized structures, the mixed mode fatigue crack growth (FCG) behavior has become a concerned problem in the field of structural integrity. In order to understand FCG behavior of I-II mixed mode crack growth at negative load ratios, an improved compact tensile shear (CTS) specimen of commercial pure titanium is taken to study the effects of load ratio and loading angle on I-II mixed mode FCG behavior through experiments and finite element method (FEM), which mechanism is discussed in depth as well. The contribution of mode I and mode II crack growth driving forces to I-II mixed mode FCG is analyzed. The changes of crack fracture criterion and the mechanism of crack growth in I-II mixed mode FCG process are revealed, and the competition relationship between two crack closure mechanisms of I-II mixed mode crack at negative load ratios is explained. An improved fatigue crack growth rate (FCGR) model that can make I-II mixed mode FCGR normalized is proposed by considering both the crack closure and the crack fracture criterion eventually.

Probabilistic analysis of fatigue crack growth using efficient surrogate model

In this paper probabilistic analysis of a fatigue crack growth process governed by the Walker law is performed. Sensitivity of its parameters to uncertainties related to the material properties is considered. Based on statistical analysis of experimental data, they are modelled as independent normal random variables. The effect of these uncertainties on the fatigue crack growth behaviour of Centre Cracked Plate specimen is assessed using Monte-Carlo simulations and surrogate model based on polynomial chaos expansion. An efficient truncation scheme allows to discard the high order interactions having a weak effect on the mechanical response and consequently reduce the number of finite elements calls when identifying the unknown coefficients of the polynomial chaos expansion. The statistical moments namely the mean and the standard deviation in addition to the probability density function of the mechanical response are derived with a good accuracy. The obtained results show that the fatigue crack growth life is significantly affected by the uncertainties on the material properties since its coefficient of variation reaches 21% and that it follows a log-normal distribution law.

The numerical simulation of fatigue crack propagation in Inconel 718 alloy at different temperatures

Inconel 718 alloy is the most commonly used material of aero-engine turbine today. However, Inconel 718 alloy is known to suffer from the fatigue crack propagation during engine operation, which will lead to unstable fracture and seriously threatens the safety of the engine. In this paper, we research the fatigue crack propagation process of Inconel 718 alloy unilateral notched standard CT specimen at room temperature (298.15 K), 573.15 K, and 823.15 K under the type I cyclic fatigue load. The ABAQUS is used for numerical simulation. First, parametric models are established. Second, the extended finite-element method (XFEM) is used to in the calculation process for describing the crack state. Third, the stress intensity factor at the crack tip under different temperatures is calculated, and the extended finite-element crack length is solved to obtain the fatigue crack growth rate da/dN curve. The innovation is we use the XEFM method to predict the fatigue crack growth process of Inconel 718 alloy, and compare it with the test results to verify the reliability of the XEFM method under low stress intensity factor. We also provide some possible reasons for the error of XEFM results under high stress intensity factors, and the development of simulation methods should be emphasized.

Shear band deformation and fatigue striations during three-point bending fatigue crack growth in brittle metallic glass

Shear band plays a key role in fatigue damage and fracture behaviors in metallic glass (MG). The effect of the shear band on fatigue crack growth of a brittle Ti32.8Zr30.2Ni5.3Cu9Be22.7 (ZT3) MG with superior fatigue resistance was investigated by three-point bending fatigue testing in this work. The fatigue crack growth process is classified into three stages based on the shear band feature and crack profile. It is found that the crack path is straight and smooth with no or several shear bands near it in the former stage of fatigue crack growth, but transformed to be staircase-like with extensive shear-band proliferation in the latter stage. The transformation of crack growth path is mainly determined by three factors, i.e., the inclined shear band with respect to mode I crack plane direction, maximum energy release rate along the mode I crack plane direction and the asymmetric shear-band attribution on two sides of mode I crack plane direction. On the other hand, the fatigue fracture surface shows three regions with different fatigue striation features, which match well with three stages on the sample surface. The shear band theory including shear-band formation, propagation and cracking is utilized to well interpret the mechanism of fatigue striation formation. The present findings reveal the relation between shear band and fatigue crack growth and may provide instructions for future design of MGs with excellent fatigue resistance through controlling shear banding behavior.

Acoustic emission analysis during fatigue crack propagation by Bayesian statistical modeling

Acoustic emission (AE) during fatigue crack growth of pure iron, three carbon steels and two Mg–Y–Zn alloys were measured and analyzed using Bayesian approaches to classify the fatigue crack growth process into several stages. By formulating the relationship between fatigue crack growth rate and AE parameters with a multiple-changepoint model and applying the experimental data to Markov chain Monte Carlo (MCMC) analysis, the fatigue crack growth process was divided into four stages. The experimental observations revealed that the classified stages I to IV correspond mainly to microstructurally small crack region, physically short crack region, stable crack growth region and unstable crack growth stage, respectively. In addition, AE parameters significant for predicting crack growth were identified at each stage by Bayesian model selection. In the microstructurally small crack region of pure iron and magnesium alloy, peak frequency and duration were selected as the significant AE parameters, respectively. In the stable crack growth region, AE event rate was selected regardless of the material. By comparing the above results with the literature, it was demonstrated that the proposed method characterizes the fatigue crack growth process by AE signals without being affected by the decision of researchers and inspectors, which is useful for understanding fatigue crack growth behavior during material testing and structural diagnosis.

Examination of Fatigue Crack Growth Process on Hydrogen Exposed Austenitic Stainless Steel by Electromagnetic Nondestructive Evaluation

Examination of Fatigue Crack Growth Process on Hydrogen Exposed Austenitic Stainless Steel by Electromagnetic Nondestructive Evaluation

Microstructure-sensitive fatigue crack growth in lath martensite of low carbon steel

The elementary process of microstructural fatigue crack growth in single-packet structures with different habit-plane orientations to the loading axis was examined using miniature compact-tension specimens to comprehend the intrinsic resistance to fatigue crack growth in lath martensite of low carbon steel. When the angle between the loading axis and the normal to the habit plane (φ) was within ~45°, the cracks propagated nearly parallel to the habit plane. In packets with their habit planes nearly perpendicular (φ > ~70°) to the notch direction, the crack grew across the laths because of damage accumulation through the activation of the out-of-plane slips, which increased the fatigue crack growth resistance. Post-fatigue-test electron microscopy revealed that the microstructural inhomogeneity of the low carbon steel enhanced the strain localisation in coarse laths, which probably led to the premature fatigue crack growth. When the crack grew perpendicular to the habit plane, the intrinsic resistance to fatigue crack growth was higher in the fine-lath region than in the coarse-lath region.

Investigation of subsurface fatigue crack growth behavior of D2 tool steel (JIS SKD11) based on a novel measurement method

In our preliminary study of the fatigue properties of D2 tool steel (JIS SKD11), it was found that owing to grinding in the specimen preparation process, high compressive residual stresses were introduced in the surface layers of the fatigue specimens, and as a result, the fatigue crack growth of the specimen was entirely subsurface. As relatively little is known about the subsurface growth of fatigue cracks, this finding gave us the opportunity to learn more about the subsurface fatigue crack growth process. An S–N curve covering fatigue lifetimes from 103 to 108 cycles was established for R = −1 loading condition. The velocity of fatigue crack growth as a function of the maximum stress intensity factor Kmax was determined to be in the range of 10−11–10−7 m/cycle. The Murakami √area method and a new experimental procedure were used to estimate the stress intensity factor and crack growth velocity under the specimen surface, respectively. An analytical equation for the velocity of crack growth as a function of the maximum stress intensity factor Kmax was developed, which agrees with the experimental results. The integration of this equation led to estimates of the fatigue lifetime for given stress ranges. These estimates were also in agreement with the experimental results.

Fatigue crack growth assessment method subject to model uncertainty

Fatigue crack growth (FCG) is an important degradation process of many critical mechanical equipment. Probabilistic FCG models are often used to account for the variability among FCG process conditions. In the well-known model of Yang and Manning, a deterministic FCG model is randomized by multiplying the crack growth rate with a random multiplier assumed to obey a lognormal distribution and unknown parameters are jointly estimated through Maximum likelihood estimation. By so doing, the modeling error due to inappropriate choice of the deterministic FCG model and that due to unsuitable assignment of the probability distribution of the random multiplier cannot be distinguished. Besides, the model uncertainty of the random multiplier is not explicitly considered. In this paper, a two-step least-square estimation method is proposed, which estimates the unknown parameters in the deterministic FCG model at first, and generates a sample set for the estimation of the random multiplier considering model uncertainty by way of Bayesian model selection. In Bayesian model selection, three types of Bayes factor are considered to select the appropriate candidate model and a simulation experiment is carried out to guide their selection. The effectiveness and feasibility of the proposed method are illustrated through two case studies using the real FCG datasets.

A research on fatigue crack growth monitoring based on multi-sensor and data fusion

Fatigue crack propagation is one of the main problems in structural health monitoring. For the safety and operability of the metal structure, it is necessary to monitor the fatigue crack growth process of the structure in real time. In order to more accurately monitor the expansion of fatigue cracks, two kinds of sensors are used in this article: strain gauges and piezoelectric transducers. A model-based inverse finite element model algorithm is proposed to perform pattern recognition of fatigue crack length, and the fatigue crack monitoring experiment is carried out to verify the algorithm. The strain spectra of the specimen under cyclic load in the simulation and experimental crack propagation are obtained, respectively. The active lamb wave technique is also used to monitor the crack propagation. The relationship between the crack length and the lamb wave characteristic parameter is established. In order to improve the recognition accuracy of the crack propagation mode, the random forest and inverse finite element model algorithms are used to identify the crack length, and the Dempster–Shafer evidence theory is used as data fusion to integrate the conclusion of the two algorithms to make a more accountable and correct judge of the crack length. An experiment has been conducted to demonstrate the effectiveness of the method.

Modeling fatigue crack and spalling for rolling die under hot milling

AbstractIn hot milling process, rolling die is subjected to nonsteady conditions which can rise the combinations of fatigue and spalling damage mechanism. An understanding about the failure mechanism of the rolling die is essential under hot rolling process. Fatigue crack growth and spalling process are governed by highly concentrated strain and stress in the crack tip region. Based on the theory of elastic‐plastic fracture mechanics, an analytical model are presented in this paper to determine the elliptical crack growth rate and spalling damage mechanism. The model includes new proposed constitutive equations for fatigue and spalling crack growth. To verify the models, finite element simulation and experimental data are considered. The results show good agreement with finite element simulation and experimental data.

The sharp drop in fatigue crack growth life at a critical elevated temperature for a PM Ni-based superalloy FGH97

The fatigue crack growth (FCG) experiments for Ni-based powder metallurgy (PM) FGH97 superalloy were performed from RT to 800 °C in air. It has been found that the FCG life decreases in an accelerated manner with an increase in temperature in the service temperature range, and a sharp drop in FCG life was noticed at the inflection point temperature (Tc) of about 800 °C. The fractographic and microstructural analyses were carried out to investigate the primary reason for the phenomenon of observed sharp drop. The obtained results indicate that the fracture transforms from transgranular to intergranular mode during the crack propagation under elevated temperature, and the transition point of fracture mode (ΔKT) also declines rapidly with an increase in temperature. At 800 °C, ΔKT appears even from the crack starts to propagate, which is closely related to the significant decline of FCG life. Combined with the calculated apparent activation energies of FCG, it can be inferred that the sharp drop in FCG life is primarily attributed to the dynamic embrittlement induced weakening of grain boundary, which dominates in the intergranular FCG stage at 800 °C. Further, the evolution of the fracture mechanism governing the FCG process under the conditions of different temperatures and stress intensity factor range (ΔK) has been discussed. With an increase in temperature in the service temperature range, the dominant mechanism of the intergranular fractures transforms from stress assisted grain boundary oxidation to dynamic embrittlement, which results in the sharp decline of FCG life at Tc.

The perturbation series method based on the logarithm equation for fatigue crack growth prediction

This paper gives the perturbation series method for the fatigue crack growth prediction accounting for the error in initial crack length. The proposed method utilizes the logarithm form of the crack growth equation and the solution of it is expressed in the form of asymptotic series by introducing a small parameter. The initial crack length is considered as a sum of the nominal value and the perturbation which mainly originates from manufacturing and measurement errors. Based on the Taylor series expansion and perturbation series method, a series of perturbation equations to predict the fatigue crack growth history are obtained. Then these equations can be solved by the successive approximation approach and the a-N curve which describes the relationship between crack length and number of load cycles can be obtained. Two examples with respect to fatigue crack growth experiments are given to verify the accuracy and validation of the presented method. The results show that the perturbation series method is closer to the experimental data compared with the original solution of crack growth equation and it can provide a more accurate prediction for the fatigue crack growth process.

Lamb Wave-Minimum Sampling Variance Particle Filter-Based Fatigue Crack Prognosis.

Fatigue cracks are one of the common failure types of key aircraft components, and they are the focus of prognostics and health management (PHM) systems. Monitoring and prediction of fatigue cracks show great application potential and economic benefit in shortening aircraft downtime, prolonging service life, and enhancing maintenance. However, the fatigue crack growth process is a non-linear non-Gaussian dynamic stochastic process, which involves a variety of uncertainties. Actual crack initiation and growth sometimes deviate from the results of fracture mechanics analysis. The Lamb wave-particle filter (LW-PF) fatigue-crack-life prediction based on piezoelectric transducer (PZT) sensors has the advantages of simple modeling and on-line prediction, making it suitable for engineering applications. Although the resampling algorithm of the standard particle filter (PF) can solve the degradation problem, the discretization error still exists. To alleviate the accuracy decrease caused by the discretization error, a Lamb wave-minimum sampling variance particle filter (LW-MSVPF)-based fatigue crack life prediction method is proposed and validated by fatigue test of the attachment lug in this paper. Sampling variance (SV) is used as a quantitative index to analyze the difference of particle distribution before and after resampling. Compared with the LW-PF method, LW-MSVPF can increase the prediction accuracy with the same computational cost. By using the minimum sampling variance (MSV) resampling method, the original particle distribution is retained to a maximum degree, and the discretization error is significantly reduced. Furthermore, LW-MSVPF maintains the characteristic of dimensional freedom, which means a broader application in on-line prognosis for more complex structures.

РАЗВИТИЕ МЕТОДА КОРРЕЛЯЦИИ ЦИФРОВЫХ ИЗОБРАЖЕНИЙ ДЛЯ ИЗУЧЕНИЯ ПРОЦЕССОВ ДЕФОРМАЦИИ И РАЗРУШЕНИЯ КОНСТРУКЦИОННЫХ МАТЕРИАЛОВ

A number of problems related to the digital image correlation method from the standpoint of both the hardware and software development and testing, as well as solving problems in the field of fatigue fracture mechanics are described. Testing of the developed computer stereo vision system was carried out using a series of stereo pairs reflecting the change in the location of the object in space as well as plane and out-of-plane deformations. It is shown that the error in determining spatial coordinates does not exceed 0.75 units, while the error in computing the strain tensor components in case of a system with a single camera is two orders of magnitude larger than that at using the stereo machine vision system. An algorithm for automatic crack detection on optical images and calculating its tip coordinates was proposed and tested. It is shown that when the frame size is 2000×1000 pixels the coordinates of the crack tip might be determined with an average error of about 56 pixels, while the average error of the crack area determination does not exceed 1.93%. A modified incremental algorithm for calculating displacements on a series of stereo pairs is proposed, which allows one to estimate large magnitude displacements during serial processing of images. An algorithm for measuring the J-integral using the digital image correlation method has been developed. It is shown that the deviation of the calculated J-integral values from the model ones is on average 1.75 %. A quantitative characterization of the fatigue crack growth process in metal alloys was carried out using the technique based on the digital image correlation (in terms of fracture mechanics) including the calculation of the fatigue crack growth rate da / dN , maximum strain (emax) and effective cycle asymmetry ( R eff).

Effects of Mg Content on the Fatigue Strength and Fracture Behavior of Al-Si-Mg Casting Alloys

In order to investigate the influence of Mg content on the fatigue properties in Al-Si-Mg alloy, uniaxial tensile tests and rotating bending fatigue tests were conducted on two Al-7Si-Mg casting alloys, with different Mg contents of 0.3 and 0.6 wt.%. The fracture surfaces of the specimens were observed by scanning electron microscopy, and the fatigue crack growth process was obtained by the replica method. The results indicate that the increase in Mg content enhances the yield strength and tensile strength of the alloy, whereas it reduces its elongation. There is an obvious knee point in the S–N curves of Al-7Si-0.6Mg alloy while that of Al-7Si-0.3Mg alloy shows continuous decrease. The fatigue strength at 5 × 105 cycles is 60 MPa for Al-7Si-0.3Mg alloy and 80 MPa for Al-7Si-0.6Mg alloy. For the two types of specimens, the fatigue failure can initiate from crystal slip and casting defects. The replica results suggest that cracks in Al-7Si-0.6Mg alloy grow more slowly than that in Al-7Si-0.3Mg alloy.